We know that the risk of getting mental illness is higher for people living in cities, explains lead author Professor Andreas Meyer-Lindenberg.

"Schizophrenia risk is doubled and there is an increase in anxiety and depression too," says Meyer-Lindenberg.

"The point of the study was to show what links [urban-city living] has to stress and mental illness", he says.

The researchers compared the brain activity of groups of volunteers who lived in urban or rural areas.

The volunteers were asked to perform a computer-based arithmetic program, but they were unaware that the program was stacked against them, giving them very difficult tasks so that they obtained low scores.

To add to the stress, an authority figure commented on their results, telling them that they were the worst performer on the task and urging them to improve.

This situation induces "social evaluative stress" where the participants feel they are failing to meet social norms, says Meyer-Lindenberg.

Lighting up the brain

The stress not only increased the volunteers' heart rate, blood pressure and levels of the stress hormone cortisol in their saliva, but also caused the amygdala, a part of the brain associated with emotion, to light up.

People living in cities showed the most activity in their amygdalas, those living in towns showed less, and people living in rural areas had least activity.

"The amygdala is the brain's danger centre," says Meyer-Lindenberg, "and it is tightly linked to anxiety and depression."

Another brain area showed activity that correlated with how much of the subjects' childhood had been spent in cities. People with a very urban childhood had more activity in the cingulate cortex.

"The cingulate is a regulatory area involved in dealing with negative life events," says Meyer-Lindenberg. "It is very vulnerable to stress early in life. It has been found to show changes in early schizophrenia."

The experiments were repeated three times and, each time, city dwellers had more brain activity.

City living not all bad

"In 1950, only 30 per cent of the world's population lived in urban regions; today, more than 50% of us do so; by 2050, this figure is expected to have climbed to almost 70 per cent," write Professor Ralph Adolphs and Dr Daniel Kennedy of the California Institute of Technology in a commentary appearing in the same issue of Nature.

They say the study shows a link, but aren't certain whether one is the cause of the other, and therefore recommend larger studies to rule out any external factors.

"Such studies could, for example, measure individuals' perceived position in a social hierarchy and frequency of encounter with strangers, as well as population density, amount of space and type of housing," they write.

They also note that there are many upsides to city living.

"In many countries, for example, studies on the complex relationship between urbanicity and suicide, show higher rates of suicide in rural areas than in cities."

"Although there are a number of possible explanations for this observation, it could relate to cities' provision of a richer, more stimulating and more interactive social environment, a larger social-support network, and easier access to medical care," they write.